Memory devices are often used in electronic systems and computers to store information in the form of binary data. These memory devices may be characterized into various types, each type having associated with it various advantages and disadvantages.
For example, random access memory (“RAM”), which may be found in personal computers, is typically volatile semiconductor memory; in other words, the stored data is lost if the power source is disconnected or removed. Dynamic RAM (“DRAM”) is particularly volatile in that it must be “refreshed” (i.e., recharged) every few hundred milliseconds in order to maintain the stored data. Static RAM (“SRAM”) will hold the data after one writing so long as the power source is maintained; once the power source is disconnected, however, the data is lost. Thus, in these volatile memory configurations, information is only retained so long as the power to the system is not turned off. In general, these RAM devices can take up significant chip area and therefore may be expensive to manufacture and consume relatively large amounts of energy for data storage.
One type of programmable semiconductor non-volatile memory device suitable for use in such systems is a programmable read-only memory (“PROM”) device. One type of PROM, a write-once read-many (“WORM”) device, uses an array of fusible links. Once programmed, the WORM device cannot be reprogrammed.
Other forms of PROM devices include erasable PROM (“EPROM”) and electrically erasable PROM (EEPROM) devices, which are alterable after an initial programming. EPROM devices generally require an erase step involving exposure to ultra violet light prior to programming the device. Thus, such devices are generally not well suited for use in portable electronic devices. EEPROM devices are generally easier to program, but suffer from other deficiencies. In particular, EEPROM devices are relatively complex, are relatively difficult to manufacture, and are relatively large. Furthermore, a circuit including EEPROM devices must withstand the high voltages necessary to program the device. Consequently, EEPROM cost per bit of memory capacity is extremely high compared with other means of data storage. Another disadvantage of EEPROM devices is that, although they can retain data without having the power source connected, they require relatively large amounts of power to program. This power drain can be considerable in a compact portable system powered by a battery.
Due, at least in part, to a rapidly growing numbers of compact, low-power portable computer systems and hand-held appliances in which stored information changes regularly, low energy read/write semiconductor memories have become increasingly desirable and widespread. Furthermore, because these portable systems often require data storage when the power is turned off, non-volatile storage devices are desired for use in such systems.
Recently, programmable metallization cell (PMC) devices have been developed for use in such systems to replace DRAM, SRAM, PROM, EPROM, EEPROM, and similar devices. PMC devices offer advantages over traditional memory devices because PMC devices can be formed using amorphous material and can thus be added to existing devices formed on a semiconductor substrate. The PMC devices also typically have lower production cost and can be formed using flexible fabrication techniques, which are easily adaptable to a variety of applications. Further, the PMC devices may be scaled to less than a few square microns in size, the active portion of the device being less than on micron. This provides a significant advantage over traditional semiconductor technologies in which each device and its associated interconnect can take up several tens of square microns.
FIG. 1 illustrates a typical PMC device 100 formed on a surface of a substrate 110. Device 100 includes electrodes 120 and 130, an ion conductor 140, and an electrode 180. Generally, device 100 is configured such that when a bias greater than a threshold voltage (VT) is applied across electrodes 120 and 130, the electrical properties of structure 100 change. For example, as a voltage V≧VT is applied across electrodes 120 and 130, conductive ions within ion conductor 140 begin to migrate and form a conductive region (e.g., electrodeposit 160) at or near the more negative of electrodes 120 and 130. As the electrodeposit forms, the resistance between electrodes 120 and 130 decreases, and other electrical properties may also change. If the same voltage is applied in reverse, the electrodeposit will dissolve back into the ion conductor and the device will return to its high resistance state.
Because PMC devices have advantages over traditional semiconductor memory devices and can be used in a wide variety of applications, improved circuits for reading, writing, and erasing PMC devices are desired. Accordingly, circuits for programming the programmable PMC devices are desired.